Sizing agents have gained wide acceptance in the paper industry for modifying paper to improve its resistance to penetration by liquids, generally water, and to improve various aspects of printability. For example, U.S. Pat. Nos. 5,122,568 and 5,139,614 teach the use of styrene acrylic copolymers for enhancing sizing, inkprint contrast, increasing ink penetration times and reducing feathering. U.S. Pat. No. 3,562,102 discloses amine reaction products of alkyl glycidyl mixed esters which are used in the preparation of sized cellulosic paper substrates, cured films and solid resinous potting compositions.
Efforts have been made to develop improved sizing agents. In U.S. Pat No. 4,855,343 acrylonitrile acrylate monomers are copolymerized by emulsion polymerization in the presence of degraded starch and a peroxide-containing initiator to provide improved paper sizing agents. U.S. Pat. No. 5,362,573 describes the use of, titanium, hafnium and zirconium salts such as ammonium zirconium carbonate in conjunction with surface sizing compound containing water-soluble hydroxylated polymer to improve sizing efficiency.
There are many different sizing agents used in papermaking, such as rosin soap, starch, sodium silicate, amine reaction products of alkyl glycidyl mixed esters, such as that described in U.S. Pat. No. 3,562,102, for example, and sizing can be accomplished by either internal sizing processes, which typically involve wet end addition, or surface sizing processes, which typically involve addition at the size press. Surface treatment at the size press typically with sizing agents alters the paper web primarily by imparting an increased degree of hydrophobicity. This sizing effect, in turn, generates higher ink penetration time values, reduced lateral spread of printing inks, as well as improved imaging and contrast with various printing technologies. Thus, hydrophobic surface size agents improve printability primarily by decreasing paper sheet absorbtivity and enhancing surface resistance to liquid (ink) penetration.
Paper printability and other qualities have also been improved through the use of coated paper and various ink jet recording sheets. In U.S. Pat. No. 4,442,172, an ink jet recording sheet made by coating a water-soluble polymer onto the surface of a base sheet containing synthetic silicate, wet strength additive and glass fiber is described as having superior ink absorbency and high recording density without feathering. Combinations of alkyl ketene dimer internal sizing agent, precipitated calcium carbonate and certain cationic synthetic fixing agents have also been incorporated into ink jet recording base layers, see, U.S. Pat. No. 5,126,010. Ink jet recording paper described in JP 06143799 is prepared by applying a surface size treatment of high molecular weight adhesive and styrene-maleic anhydride copolymer to both sides of base paper and disposing an ink-receiving coating containing a white pigment and adhesive on one side of the paper. Ink jet recording transfer sheets containing calcium carbonate as a filler and alkyl ketene dimer or alkenyl succinic anhydride as sizing agents are described in JP 06008617. Auxiliary agents such as starch, polyacrylamide and polyvinylalcohol may be included in the transfer sheet. J56109783-A describes paper for ink jet recording obtained by applying a hydrophilic polymer, e.g., oxidized starch or polyvinyl alcohol, to raw paper of specified water absorption degree to provide paper with balanced ink drying properties and blotting resistance for ink-jet printing. And U.S. Pat. No. 4,900,620 describes ink jet recording sheets having good ink absorption, resolution and color brightness which are made of wood pulp and precipitated calcium carbonate substrate coated with a layer of white pigment. Ink jet recording sheets composed of a support and a cation-modified acicular or fibrous colloidal silica-containing ink-receiving layer are described in U.S. Pat. No. 5,372,884; various polymers may be included in the ink receiving layer for improving ink drying properties, film-forming properties and image sharpness.
In U.S. Pat. No. 4,294,704 paper and board is coated with compositions containing aqueous latex binders of synthetic polymer such as styrene-butadiene copolymers, carboxyl styrene-butadiene copolymers, vinyl polyacetates, carboxyl vinyl polyacetates, alkyl acrylate-vinyl acetate copolymers and carboxyl alkyl acrylate-vinyl acetate copolymers in order to provide improved dry and wet tear resistance. In U.S. Pat. No. 4,439,496 a water-resistant photographic substrate containing an alkylketene dimer, a cationic polyacrylamide and an anionic polyacrylamide is coated with a water resistant polyolefin.
JP05173287 discloses base paper for use in photographic printing paper which is made by adding a cationic polyacrylamide to pulp slurry, then adding a sizing agent which has an effect in the weakly acidic to weakly alkaline pH region, e.g., high organic ketene dimers, and finally adding an anionic polyacrylamide; the base paper is described as being useful in inhibiting the permeation of developing liquids from the cut end surfaces of the photographic printing paper. Another ink jet recording sheet is described in EP 600245.
As new printing technologies continue to emerge and gain acceptance in the market place, the need for developing high performance paper surfaces having improved printability characteristics increases. Despite the benefits obtained from using coated paper or ink jet recording sheets, these materials have certain disadvantages, for example, they require two separate layers and therefore their manufacture involves numerous paper processing steps and special converting operations. Furthermore, many of the existing coated paper sheets exhibit inadequate color image, color resolution and color bleed. Moreover, as paper products increasingly use multiple colored inks, paper exhibiting controlled ink absorption, optimal color density and reduced color bleed, and with reduced feathering and wicking has been particularly difficult to make. It is desirable, in many instances, to use single sheets of paper having superior color absorption rates, color density and resolution with reduced color bleed and feathering.
Consequently, a need, unsatisfied by existing technology, has developed for a paper additive that provides the above-described benefits in printability and sizing yet also exhibits superior dry strength.
Existing dry strength additives have not met this need. N-(chloroalkoxymethyl) acrylamides and quaternized derivatives thereof are dry strength additives described in U.S. Pat. No. 3,980,800. Canadian patent 1101158 describes improved dry strength properties obtained by admixing a fiber suspension with quaternized acrylamide polymers. The addition of acetone-triethylenetetramine-diethylenetriamine formaldehyde copolymer and acrylamide-maleic anhydride copolymer to cellulose pulp is described in U.S. Pat. No. 4,036,682 as giving increased strength properties. Polyvinyl alcohol and starch have been applied to paper to improve sheet surface characteristics which may included surface strength, and porosity smoothness. In JP 57139599 and JP 139598 anionic polyacrylamide is added at the wet end and a sizing agent selected from alkyl ketene dimer, cationic sizing agent and substituted cyclic dicarboxylic anhydride is used along with aluminum salt to manufacture neutral paper purportedly having good wet and dry tenacity.
Various printing technologies continue to require paper surfaces with high integrity and resistance to the abrasive effects of the printing processes. Fiber picking, linting and filler dusting all contribute to decreased runnability of paper on impact printing processes. There therefore exists a need for surface treatment agents which overcome these surface defects.
It has now been discovered that a generally hydrophobic surface size additive and a hydrophilic dry strength agent, preferably polyacrylamide, may be combined in a single composition to provide, when added to paper or board sheet surfaces at the size press, synergistically improved sizing and strength as well as a wide range of improved paper printability properties. It was surprising to find that a hydrophilic agent such as polyacrylamide could be combined with a hydrophobic surface size as a pre-mix and that improved strength, sizing and printability properties resulted from use of that pre-mix in paper or board including paperboard. This was particularly unexpected given that hydrophobic surface size agents, in theory, increase the degree of hydrophobicity of the sheet and thus decrease paper absorptivity while hydrophilic materials such as polyacrylamide increase the degree of hydrogen boding in the paper sheet and maintain high sheet absorbitivity of liquid. Paper or board made using the compositions of the instant invention exhibit sizing properties which are superior to the use of the hydrophobic surface size agent alone. Thus, by admixing polyacrylamide with a surface sizing agent the sizing function is unexpectedly improved. A further benefit provided by the compositions of the instant invention is synergistically improved strength properties, such as internal bond strength and wax pick. In addition, paper and board comprising the surface size polyacrylamide mixtures of the instant invention exhibit synergistic improvements in print quality, e.g., controlled ink absorbency, ink drying time and color optical density; sharper half-tone dots are produced with maximized light scatter for improved print contrast. The products of the instant invention impart substantial resistance to penetration of ink and aqueous liquids to said paper, provide processing flexibility by allowing the papermaker to balance the size requirements between internal and surface sizing, increase the contact angle and surface strength, maintain the sizing level over time and impact the coefficient of friction. Other benefits relating to printability such as improved image resolution and reduced color bleed, bleed through, feathering, wicking, picking, Tinting and dusting are also achieved by the instant invention. The superior image resolution and bleed through properties provided by the compositions of the instant invention are particularly desirable for use in connection with modern printing technologies which use a variety of different color inks. Advantageously, the above benefits are provided by a single sheet of uncoated paper or board without having to produce a more complicated and costly multilayer paper or board, e.g. coated paper or base paper with an ink receiving layer.
The surface size materials used in the instant invention are generally synthetic, preferably, though not necessarily, water soluble, and are primarily hydrophobic, preferably containing at least about 30 mole percent hydrophobic groups, while the acrylamide polymers with which they are combined are substantially hydrophilic, preferably containing at least about 25 mole percent, more preferably at least about 50 mole percent hydrophilic monomer groups such as the amide group in polyacylamide. The surface size and polyacrylamide material must be premixed prior to application to the paper. The polyacrylamide surface size mixtures of the instant invention may be added alone as a water solution in dispersion or in combination with commonly applied paper additives such as size press starches. The compositions of the present invention may be used in the production of board such as boxboard, linerboard and bleached board, e.g. milk carton, as well as various types of paper such as, for example uncoated, woodfine papers, packaging, newsprint, and ledger. The paper and board of the instant invention is especially useful for paper or board which is designed to be or capable of being printed in its end use, i.e. printing paper or board.
In yet a further embodiment of the present invention compositions comprising a hydrophobic surface size agent, a hydrophilic polyacrylamide and a cross-linking agent, such as, for example, ammonium zirconium carbonate, are provided and when applied to paper or board, at the size press, or the like, further improve printability, both non-impact and impact printability, internal dry strength, and surface strength, e.g. tensile strength, burst, and wax pick.